Magnetic tape recording systems are widely used to archive digital information, but have historically been plagued by performance and storage problems that render their continued use for high volume information storage unacceptable. For example, due to the relatively low storage density of the magnetic recording media, a large number of expensive tape reels or cassettes are required to store the information. Furthermore, the mechanical devices and parts used in providing a storage system for the reels and cassettes of recorded media often require expensive and time consuming maintenance and/or complete replacement. The magnetic tapes must also be repacked every six months to account for tape stretch and rerecorded every five to ten years in order to preserve data integrity.
Optical systems are now commonly used in place of magnetic systems for recording and playback of digitized information. In optical recorders, the data is used to amplitude modulate a light beam having a predetermined intensity necessary to mark a light sensitive recording media. The modulated beam is focused to a small spot and traced across the media to record the data as a fine optical pattern comprised of a number of closely spaced, microscopic dots (data marks) along a data track. To recover the recorded data from the optical media, a low intensity illumination beam is scanned along the data track and modulated by the optical pattern recorded therein. The modulated beam is reflected from the media to illuminate a light detector producing an electrical signal in accordance with the beam modulation for recovery of the recorded data.
Optical recording and playback systems have proven to provide enhanced performance characteristics over magnetic systems. The microscopic optical pattern of data recorded on the recording media by an optical system dramatically increases the data storage density over conventional magnetic systems. Furthermore, there is a decreased susceptibility to tape stretch and wear with optical playback systems because, unlike magnetic systems, there is active optical tracking during the reading process with no contact between the read head and the media. Accordingly, data life on the media is increased to over twenty years. Finally, because significantly more data can be stored by optical systems, the complexity of an archiving system (and required space) for storing recording media reels and cassettes is significantly reduced.
The problem with present optical recording and playback system technology, however, is low record and playback data rates. Currently available "high speed" optical recording systems provide only a three megabyte (twenty-four megabits) per second record or read rate. At that rate, it would take nearly four full days to record or playback one terabyte of digital data. For current and future needs, record and playback rates on the order of three megabytes per second are unacceptably slow. The National Aeronautics and Space Administration, for example, anticipates a data rate of 500 megabits per second by the year 2000 for the Deep Space Network. This is far too much data, on the order of a five terabyte per day archive rate, for conventional optical systems to handle. Accordingly, there is a need for an improved optical data storage and playback apparatus capable of handling data input and output rates in excess of 400 megabits per second.